1. Field of the Invention
The present invention generally relates to disk apparatuses and disk cartridges, and more particularly a disk apparatus configured to conduct recording and/or reproducing information onto/from a disk, and a disk cartridge configured to accommodate the disk. 2. Description of the Related Art
Optical disks that store mass digitized data in response to, for example, the recent digitization of television broadcasting are in demand. A basic method of improving recording density on the optical disks may be to reduce the diameter of a beam spot used for recording/reproducing information onto/from the optical disk.
For this reason, it is effective to shorten the wavelength of light used for recording/reproduction, and also, it is effective to enlarge the numerical aperture NA of an objective lens. As for the wavelength of light, the red light wavelength of approximately 650 nm is used on a DVD (digital versatile disk) while the near infrared light wavelength of approximately 780 nm is used on a CD (compact disk). Recently, a semiconductor laser of blue-violet light has been developed and it is expected that a laser light of approximately 400 nm will be used.
The objective lens for a CD has less than 0.5 NA while the objective lens for a DVD has approximately 0.6 NA. The numerical aperture (NA) must be enlarged further in the future.
However, enlarging NA of the objective lens and shortening the wavelength of light may result in making the depth of focus of an optical system including the objective lens smaller. Because of this, it will be necessary to increase focus servo accuracy of the optical disk apparatus. Moreover, when the objective lens having a larger NA is used, the distance between the objective lens and a recording surface of the optical disk becomes shorter. Thus, surface vibration of the optical disk should be reduced or a pickup problem of a crash between the objective lens and the optical disk may occur just before the focus servo is activated.
Under the above-discussed background, an optical disk where a recording surface is formed on a flexible sheet (thin film) has been developed. A disk apparatus wherein a disk is rotated on a stabilizing board so that surface vibration of the disk is aerodynamically suppressed is proposed. See, for example, Japanese Patent Application Publication No. 53-42807 and “Optical Readout of Videodisc”, IEEE Transaction on Consumer Electronics, November, 1976, pp. 304-308.
A disk apparatus disclosed in Japanese Patent Application Publication No. 57-33619 has a stabilizing member called a back plate formed by bending both ends of a circular plate. When the disk is rotated for recording or reproducing information, the back plate is pushed toward a recording surface of the disk so that surface vibration in the vicinity of a head is suppressed. However, in this method, a peculiar flexing operation is required of the optical disk, following a bending part defined by a plane surface forming a recess surface. Hence, such a peculiar flexing operation may not follow due to high speed of rotation.
On the other hand, a disk apparatus disclosed in “Optical Readout of Videodisc”, IEEE Transaction on Consumer Electronics, November, 1976, pp. 304-308, has a stabilizing board called a saddle plate and a stabilizing member called a U-shaped stabilizer. When information is recorded or reproduced onto or from the disk, the disk is rotated so that the disk is clamped in a gap of the U-shaped stabilizer. In addition, a negative pressure is generated by using the saddle plate. As a result of this, the disk is curved in a direction opposite to that of the disk apparatus disclosed in Japanese Patent Application Publication No. 57-33619 so that the surface vibration is selectively stabilized in a specific area in the gap. However, in this method, if the rotational speed is made high, the negative pressure generated by the saddle plate may not be able to resist a centrifugal force applied to the disk so that the disk may not have a desirable curved configuration and the surface vibration in the specific area in the gap may increase.
Meanwhile, Japanese Patent Application Publication No. 57-33619 describes that a stabilizing member having a curved surface (hereinafter “continuous surface”) which is evenly curved is not proper for stabilizing a flexible optical disk medium. However, inventors of the present invention studied and found that this continuous surface effectively acts in stabilizing the flexible disk in an area where the rotational speeds exceeds 4000 rpm. In addition, the inventors of the present invention realized that, in a stabilizing member having a simple continuous surface, (i) in a case where the continuous surface is cylindrically curved concave shaped against a disk, as compared to a surface vibration property in the vicinity of a generating line passing a center of the continuous surface, a surface vibration property at its downstream side in a rotational direction is more stable; and (ii) in a case where the continuous surface is convex shaped against a disk, as compared to a surface vibration property in the vicinity of a generating line passing a center of the continuous surface, a surface vibration property at its downstream side in a rotational direction is more stable.